How do you visualize Decision boundaries?

To visualize a decision boundary of a classifier, specifically a binary classifier as in your case, you can instantiate a grid of points that spans the domain of interest that you want to classify. Then use your trained classifier to predict the class of each and every point in the grid. If the grid resolution is fine enough, when you plot the contours of the grid points with color corresponding to its class, you can then visualize the decision boundary. It is helpful to overlay the original training data points so that you can see how your decision boundaries are generated based on various training sets. See the following example